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MEEF - Recycling Technologies  - previous page

 


How is plastic recycled?

In the United States 75 billion pounds of plastic are produced every year, unfortunately the majority of this plastic ends up in landfills. When plastic is dumped into landfills the decomposition process can take anywhere from 10 to 30 years. Recycling has therefore become a reasonable solution to the landfill problem.

There are five factors that are necessary in order for the recycling of plastic to be a successful process. First, the supply of used plastic has to be of a large quantity. This large quantity of plastic is collected at certain areas, which is the second step. Once the plastic is collected, the sorting and separating process begins; this is the third step in the process. The sorting and separating process depends upon the type of polymers that make up the plastic. Plastic products are given codes to help the sorting and separating process. The fourth step in plastic recycling is reprocessing. The reprocessing of polymers includes the melting process, the melting process can be accomplished if the polymers have not been widely cross-linked with any synthetics. If the cross-linking of polymers contain too many synthetics, the polymers will be difficult to stretch and less pliable. The final step is the manufacturing of the melted plastic into new products.

The codes on plastic recyclable containers are what help most in the sorting and separating process. The six categories of plastics are separated into two areas: polyethelyne plastics and polymer plastics. The polyethelyne plastics are labeled HDPE, for high density polyethelyne; or LDPE, for low density polyethelyne. The four polymer plastics that are recycled include polyvinyl chloride, labeled V; polystyrene, labeled PS; polypropylene, labeled PP; and polyethylene terephthalate, labeled PETE. These names and labels can seem confusing, but they are a necessity in the recycling process.

There are four types of recycling processes that usually occur: primary, secondary, tertiary, and quaternary. The primary recycling process is recycling materials and products that contain similar features of the original product. This process is only feasible with semi-clean industrial scrap plastics, therefore this process is not widely used. Secondary recycling allows for a higher mixture of combination levels in plastics. When the secondary process of recycling is used it creates products such as fenceposts and any products that can be used in the substitution of wood, concrete, and metal. The low mechanical properties of these types of plastics are the reason why the above products are created. Tertiary recycling is occurring more and more today because of the need to adapt to the high levels of waste contamination. The actual process involves producing basic chemicals and fuels from plastic. The last form of recycling is the quarternary process. This quarternary process uses the energy from plastic by burning. This process is the most common and widely used in recycling. The reason this process is widely used is because of the high heat content of most plastics. Most incinerators used in the process can reach temperatures as high as 900 to 1000 degrees Celsius. For the sake of the environment the new techniques being used with the incinerators have decreased the amount of air pollutants being released.

The use of incineration in the quarternary process is most beneficial because through the high temperature heating process the incoming waste is reduced by 80% in weight and 90% in volume. The materials left over form this process are then placed in landfills.

Why Worry About Recycling Plastics?

A current promotional program sponsored by the plastics industry emphasizes the positive contributions that plastics make. And claims listed in those advertisements are accurate.

But as shown in the article on the preceding page, the largest single use for plastics is packaging. Because packaging has a short lifespan, it makes up a large portion of the plastics waste stream. But where does that “waste stream” lead?

In general, the Environmental Protection Agency says that in the early 1990s about 80 percent of all municipal solid waste was sent to landfills, 10 percent was incinerated and 10 percent was recycled. While more and more plastic is being recycled, the EPA estimates that plastics make up about 20 percent of the solid waste that is landfilled.

Most consumers think that the slow degradation of plastics is the primary reason that plastics should be recycled. However, research has shown that other waste, such as paper, wood and food wastes, also degrade very slowly in landfills.

The more serious problem with plastic waste concerns the additives contained in plastics. These additives include colorants, stabilizers and plasticizers that may include toxic components such as lead and cadmium. Studies indicate that plastics contribute 28 percent of all cadmium in municipal solid waste and about 2 percent of all lead. Researchers don’t know whether these and other plastic additives contribute significantly to products leached from municipal landfills.

How toxic are plastics that are burned? Researchers don’t know that, either. Plastics that contain heavy-metal-based additives may also contribute to the metal content of incinerator ash. The EPA is looking for substitutes for lead- and cadmium-based additives.

One additional concern relates to use of petroleum products. All plastics began their lives as petroleum. By increasing plastics recycling, scientists and engineers are able to reduce dependence on petroleum.

First Things First: Sorting

Before plastic waste can be converted into new products, the various types of plastics must be separated. Initially plastic reclamation companies relied on manual sorting — either by consumers themselves or by paid workers — but manual sorting is considered too unreliable and too expensive.

At least two organizations have developed systems for automated plastics sorting. National Recovery Technologies received the EPA’s Small Business of the Year National Award in 1991 for its efforts in developing and marketing a high-speed, automated system that efficiently separates vinyl containers (those marked #3) from mixtures of whole or crushed post-consumer plastic containers. NRT says that the presence of chlorine atoms within vinyl resins triggers a computer-timed air burst that separates vinyl containers from the mixed plastic stream. The company also developed a system that optically scans mixed plastics to separate PET soda bottles from HDPE milk jugs, green PET
from clear PET, as well as other specifications.

Sandia National Laboratories, which works with the U.S. Department of Energy, has designed a device to classify plastic waste into one of  the seven plastics categories. Near-infrared light is used to distinguish one plastic from another using the vibrational characteristics unique to each. Sandia engineers report that the device can classify many types of plastics with a success rate of 98 to 100 percent. The laboratory has issued a license for commercial development of this new device.

Old ABS Phone Housings Recycled Into Innovative Mounting Panels

Seeking new uses for recycled plastic from old telephones, AT&T Bell Laboratories engineers are remolding discarded phone housings into mounting panels for AT&T’s business telephone systems and improving service to business customers as well.

“Until now, when a telephone reached the end of its life, AT&T would sell the plastic to a recycler who would grind it up and resell it into the secondary market, where it was made into products ranging from tape cassettes to park benches,” said Werner Glantschnig, a member of Bell Laboratories technical staff and the project’s leader.

“However, we wanted to see if we could close the loop ourselves and re-use these millions of pounds of ‘ABS plastic flake’ in a way that makes both environmental and business sense.”

ABS — or acrylonitrile-butadiene-styrene — plastic flake can’t be made into new telephones because colors change during re-melting, and the plastic loses the smooth, glossy finish AT&T requires for its phone housings.

“When we mold the ABS into telephone system mounting panels, the colors disperse nicely into a uniform gray and the finished product meets all of our requirements,” said Louis D’Anjou, another Bell Laboratories engineer and the panel’s designer.

With these ABS panels, AT&T’s supply centers can now assemble and test business telephone systems before delivery to the customer’s premises. Previously, these panels had to be custom-made from plywood and the system assembled and tested at the customer’s location. In addition to reducing the use of wood, the new method is far more efficient, reducing both the time and cost of installation.

AT&T engineers and designers are looking into other possible uses for the ABS plastic flake, including spools for copper and fiber optic telephone cables.

“This is encouraging evidence that environmental awareness and the concept of ‘design for environment’ are spreading through the AT&T design community,” said John C. Borum, AT&T environment and safety engineering vice president. “Our goal is to remain in the vanguard of environmentally responsible corporations by recycling as many of our products as we can.”

Junk Car Seats Lead To New Technology

Throughout the environmental movement, researchers are concerned with products that are discarded in large quantities. Junk cars are one such product. But junk cars pose unique problems because of the combination of products used in automaking. Used metals from junk cars have long been recycled, and now many researchers are turning their attention to other auto components.

The Center for Excellence in Polymer Science and Engineering at the Illinois Institute of Technology has focused one project on the 400 million pounds of polyurethane foam scrapped each year from junk cars. IIT has patented a solid state sheer extrusion process and apparatus that could be used to recycle that waste, along with a broad range of other polymer wastes and rubbers.

IIT’s system, known by the acronym SSSE, pulverizes polymeric material, producing fine powders that have numerous applications for industry. The advantage of SSSE is that it can be applied economically to many types of natural and synthetic polymer wastes. The Center for Excellence in Polymer Science and Engineering points out that many recycling processes developed to date have been limited to certain types of waste. Most processes have not been economical, especially in the amount of energy needed, they add, and the reclaimed materials have not been produced in forms that are needed and usable for re-manufacturing.

The SSSE technology has been optioned to a New York firm for possible development. More information is available through IIT’s Office of  Public Relations, (312) 567-3104.

Research Into Chemical Recycling Could Open New Opportunities

The U.S. Department of Energy conducts on-going research on plastics recycling. This report highlights new approaches to chemical recycling. Recycling of plastics can be costly and difficult because of constraints on waste contamination and inadequate separation prior to recycling. Chemical recycling could remove some of those restraints.

Pyrolysis and hydrolysis are two processes that have shown promise in the recovery of basic chemicals and fuels from waste plastics. Pyrolysis is a process in which plastic wastes are heated in the absence of oxygen in a closed chamber. The products of pyrolysis may be used as a chemical feedstock or fuel. Hydrolysis decomposes plastic wastes through a series of chemical reactions.

Research sponsored by the U.S. Department of Energy’s Office of Industrial Technologies at the National Renewable Energy Laboratory has led to the development of a new process based on the pyrolysis of certain waste streams. This process retrieves monomers, the basic building blocks of a polymer, and high-value chemicals that are sufficiently pure to use in making new plastics. The advantage of this process is that the waste plastics do not have to be separated ahead of time, thereby eliminating a labor-intensive step in current processes. It also will reduce the cost of the monomers and chemicals and will reduce consumption of petroleum, the source of chemical feedstocks used to produce plastics.

In the new process, monomers and high-value chemicals are retrieved from manufacturing or post-consumer wastes through sequential pyrolysis. The reaction products undergo detailed chemical analysis to determine conditions that allow control of pyrolysis reactions. This allows the design of a process to collect the desired products in high yields, reducing requirements for subsequent separation and purification of the target product. NREL has filed patent applications to cover the process for a total of seven mixed plastic waste streams.

For example, NREL has demonstrated the new process of waste carpet recycling. Caprolactam, the valuable monomer of Nylon 6 used in about half of all carpet fibers, can be isolated with yields of 85 percent. This can be done without separating the nylon from the backing material.

An economic evaluation of recycling caprolactam performed by an independent economic firm shows the applications to be promising. Those findings project that a commercial-size plant recycling 100 million pounds of waste carpet could produce high-grade caprolactam for about 15-50 cents per pound. The chemical currently sells for 90 cents to $1 per pound. In other words, recycling caprolactam could reduce its cost by 50 percent or more.

Other applications of the chemical recycling process include recovering terephthalic acid from polyethylene terephthalate, or PET, in mixed plastic bottles and recovering styrene from mixed residential plastics. PET recycling does not have as favorable economics as the polyurethane application because of the lower value of plastic bottles but the potential volume of the waste stream is very large. Researchers estimate that 900 million pounds of thermoplastic polyester resin, of which PET is a major component, could be recycled each year.

Researchers are expanding the technology base for the chemical recycling process and are identifying new, promising applications for specific waste streams. Experiments are currently underway using engineering-scale reactors to confirm process reactions and to refine operating conditions.

Do-It-Yourself Plastic: Gloop

Here’s a simple activity to share with students to help them understand some characteristics of plastics and other polymers. It was developed for the Sandia National Laboratory “Science and Math Carnival.” This activity is suggested for students in third through eighth grade.

Gluep is a polymer made from borax (sodium tetraborate) and white glue. Each of these materials is a polymer already. Glue is a mixture of polyvinyl acetate and polyvinyl alcohol. Borax forms long borate chains in an aqueous solution. When the two materials are mixed together and the mixture is kneaded by hand, crosslinking of the polymer chains occurs as a result of hydrogen bonding with water molecules which links the two polymer chains. The physical properties of the mixture are quite different from the properties of the individual compounds. The resulting Gluep is a semi-solid plastic-like material.

Materials

     4 percent borax solution (1/4 cup of borax dissolved in 1 quart of tap water)
     White glue mixture (50:50 mixture of glue and water, mixed well)
     Food coloring
     Ziploc plastic bags

Procedure

  1. Pour 15 ml (1 tablespoon) of the borax solution into the bag.

  2. Add three drops of food coloring.

  3. Add 60 ml (4 tablespoons) of the white glue mixture.

  4. Zip the plastic bag tight.

  5. Knead thoroughly until the color is uniform and water is no longer visible. Consistency should be reached within 10 minutes.

  6. Remove Gluep from the bag by turning the bag inside out and rubbing the Gluep from the sides.

  7. Store the Gluep in the plastic bag.

Questions

How is Gluep like a solid? Like a liquid? What happens if you leave Gluep out of the bag? What happens if you freeze it?

Extension

Try other experiments with the Gluep. What happens when extra borax solution is added? What happens when extra glue is added? What happens if a base or acid is added during mixing? After the Gluep has hardened?

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